Preparation of stable, bright luminescent nanoparticles having compositionally engineered properties

ABSTRACT

A method is provided for preparing luminescent semiconductor nanoparticles composed of a first component X, a second component A, and a third component B, wherein X, A, and B are different, by combining B with X and A in an amount such that the molar ratio B:(A+B) is in the range of approximately 0.001 to 0.20 and the molar ratio X:(A+B) is in the range of approximately 0.5:1.0 to 2:1. The characteristics of these nanoparticles can be substantially similar to those of nanoparticles containing only X and B while maintaining many useful properties characteristic of nanoparticles containing only X and A; and can additionally exhibit emergent properties such as a peak emission energy less than that characteristic of a particle composed of XA or XB alone. This method is particularly applicable to the preparation of stable, bright nanoparticles that emit in the red to infrared regions of the electromagnetic spectrum.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of: U.S. application Ser. No.12/484,836, filed Jun. 15, 2009, now U.S. Pat. No. 8,287,761, which is acontinuation of U.S. application Ser. No. 11/011,827, filed Dec. 13,2004, now U.S. Pat. No. 7,695,642, which claims priority under 35 U.S.C.§119(e)(I) to U.S. Provisional Application Ser. No. 60/529,058, filedDec. 12, 2003, the disclosures of which are incorporated by reference intheir entirety.

TECHNICAL FIELD

The present invention relates generally to luminescent nanoparticles,and more particularly relates to the preparation of luminescentsemiconductor nanoparticles in a manner that enables compositionaltuning of electronic and optical properties without a change in particlesize or a significant change in composition.

BACKGROUND

Semiconductor nanocrystals, or “quantum dots,” are particles whose radiiare smaller than the bulk exciton Bohr radius and constitute a class ofmaterials intermediate between molecular and bulk forms of matter.Quantum confinement of both the electron and hole in all threedimensions leads to an increase in the effective band gap of thematerial with decreasing crystallite size. Consequently, both theoptical absorption and emission of semiconductor nanocrystals shift tothe blue (higher energies) as the size of the nanocrystals gets smaller.

Quantum dots are composed of an inorganic, crystalline, semiconductivematerial and have unique photophysical, photochemical, and nonlinearoptical properties arising from quantum size effects, and have thereforeattracted a great deal of attention because of their potentialapplicability in a variety of contexts, e.g., in biological detection,light-to-chemical or light-to-electrical energy conversion schemes,catalysis, displays, and telecommunications. Quantum dots arecharacterized by size-dependent properties such as peak emissionwavelength and quantum yield. These crystals generally vary in size fromabout 1 nm to 100 nm and may be variously composed of elements, alloys,or other compounds. The desirable properties of quantum dots differdepending on the field of use, but a “tunable” peak emission wavelength,chemical stability, and photochemical stability are generally viewed asvery important regardless of context.

For emission in the visible region of the electromagnetic spectrum,cadmium selenide (CdSe) materials have by far been the most importantclass of quantum dots, largely because they exhibit size-dependentluminescence tunable throughout the visible wavelength range. That is,by changing the particle size of CdSe quantum dots, the emission can bevaried throughout most of the visible wavelength region. Properselection of synthetic conditions furthermore allows the preparation ofexceptionally bright quantum dots with luminescence efficienciesapproaching unity (i.e. one emitted photon for every absorbed photon).Unfortunately, these CdSe-based quantum dots suffer from less than idealstability characteristics, particularly with regard to chemicaldegradation and photooxidation. Emission from the nanocrystals is fairlyeasily and irreversibly quenched under conditions common in, forexample, biological assays and biomedical labeling applications.

A key innovation that has significantly increased the utility of quantumdots is the addition of a discrete inorganic shell over the nanoparticlecore. That is, decomposition pathways in many quantum dots, includingCdSe nanocrystals, usually involve the formation of defects known astraps on the surfaces of the quantum dots. The key to ensuring andmaintaining quantum dot emission is to passivate these surface sites.Some have had reasonable success in passivating nanocrystal surfacesusing organic capping materials such as an alkylamine ortrioctylphosphine oxide, but thus far these approaches have proveninadequate, dramatically decreasing luminescence intensity and resultingin nanoparticles that are insufficiently robust for many applicationsincluding biological detection. The use of inorganic compounds ascapping agents has proven far more successful, providing that thematerial used is optically non-interfering, chemically stable, andlattice-matched to the underlying material. This last property isparticularly important, since matching the lattices, i.e., minimizingthe differences between the shell and core crystallographic structure,minimizes the likelihood of local defects, shell cracking, and formationof long-range defects. Typically, a large band gap semiconductingmaterial such as zinc sulfide (ZnS) will be used to epitaxially overcoatnanocrystal cores with a crystalline shell that matches the underlyinglattice. In other words, crystalline growth of a core material such asCdSe can be halted and then continued using a related crystallinematerial such as ZnS to form the shell. While this outer materialdoesn't necessarily contribute directly to the size-tunable propertiesof interest such as peak emission wavelength, such a passivating layercan have a substantial indirect impact. For example, the brightness ofcore-shell materials often far exceeds that of base nanoparticle corematerials. Additionally, resistance to chemical and photochemicaldecomposition is often markedly increased.

Though not often recognized, such shell chemistry can be criticallyimportant to the utility of quantum dots, at least in applications thatrequire certain stable properties such as predictable non-fluctuatingemission characteristics. Indeed, it has been the invention of thecore-shell concept that has resulted in recent attempts to commercializequantum dot technology in several fields, including biotechnology andsolar energy applications.

Not only is the composition of the shell of central importance to thecharacteristics of the final quantum dot, but the method of depositingthe shell material is important as well. High quality inorganic shellsmust be thick enough to be sufficiently protective, and, ideally, areintimately wed to the underlying core. The reason for the latterrequirement is that core crystals and shell crystals seldom havecompletely matched lattice spacings. For example, with a CdSe/ZnSnanoparticle as described above, the ZnS shell is characterized byshorter average bond lengths than in the CdSe core. In order tosuccessfully form high quality composite structures, special precautionsmust be taken, e.g., doping of atoms from the core into the shell torelax the lattice in the shell and allow it to more easily match thelattice of the core. The use of these alloyed or mixed shells has beendescribed in U.S. Pat. No. 6,815,064 to Treadway et al., assigned toQuantum Dot Corporation (Hayward, Calif.) and incorporated by referenceherein.

Despite the significant commercial impact of the new engineerednanoparticle structures, there remain limitations in the field whichhave not yet been overcome. For example, it is now understood that manyapplications demand the ability to independently tune the size and theemission characteristics of the final nanoparticles rather than allowingthem to move in lockstep. One example of this need is related to thefact that the efficiency of light absorption, and therefore the ultimatebrightness, of the nanoparticle is a steep function of the particlesize. It is desirable therefore in some applications requiring extremelysensitive detection to maximize the size of the nanoparticles withoutnecessarily maximizing the emission wavelength for the resulting labels.Alternatively, some potential uses for quantum dots require them to beprepared as small as possible, particularly where steric or otherphysical constraints limit the size of the label which can be used(e.g., labeling inside the nuclei of living cells). Again, it isdesirable to prepare a palette of colors for this application, but heresmaller particles are more useful.

Another limitation which must be overcome is the fact that the cost ofresearch associated with the development of high-quality inorganicpassivating layers for nanoparticles is extremely high. Passivation iscritical to most quantum dot applications, if not all applicationsrequiring luminescent versions of the particles. For this reason, it isuseful to make a single overcoating material (e.g., ZnCdS) serve formany distinct nanocrystal core compositions.

It has further proven difficult to engineer the peak emission wavelengthof a luminescent semiconductor nanoparticle, e.g., to provide emissionin the red to infrared regions of the spectrum, without increasingparticle size.

Some attempts have been made to engineer the electronic and opticalproperties of luminescent semiconductor nanoparticles by dopingnanoparticle cores with an additive. For instance, a method has beendescribed for increasing peak emission wavelength by doping nanoparticlecores with a material capable of shifting the emission peak to thedesired extent, but the resulting core-shell structures exhibit rapiddegradation of optical properties. See, e.g., Zhong et al. (2003),“Composition-Tunable Zn_(x)Cd_(1-x)Se Nanocrystals with HighLuminescence and Stability,” J. Am. Chem. Soc. 125:8589-8594. This is inlarge part because the amount of dopant believed necessary to effect asignificant change in peak emission wavelength was so high, and anintroduction of a substantial amount of dopant changed the properties ofthe core adversely. For example, it has been disclosed that doping CdSewith Te at a level of at least 50% (such that the molar ratio of Te toSe in the CdSe_(1-x)Te_(x) core is greater than 1:1) is necessary toprovide a meaningful increase in peak emission wavelength. See Bailey etal. (2003), “Alloyed Semiconductor Quantum Dots: Tuning the OpticalProperties without Changing the Particle Size,” J. Am. Chem. Soc.125:7100-7106.

There remains, accordingly, a need for a way to prepare luminescentnanoparticles in a manner that enables engineering of key electronic,optical, and physical properties, e.g., bandgap energy, brightness, peakemission wavelength, chemical stability, and photochemical stability,without necessarily increasing particle size or significantly changingthe composition of the nanoparticle core. Such a method would be highlyvaluable in many contexts, for example enabling preparation of bright,stable particles emitting in a longer wavelength region—e.g., in the redto infrared regions of the electromagnetic spectrum.

SUMMARY OF THE INVENTION

The invention is addressed to the aforementioned need in the art, andprovides a way to engineer the electronic and optical properties of aluminescent semiconductor nanoparticle without increasing particle sizeor adversely affecting key properties of the nanoparticle, particularlybrightness, chemical stability, and photochemical stability. Theinvention is premised on the discovery that the electronic and opticalproperties of a luminescent semiconductor nanoparticle can becompositionally engineered, i.e., shifted by a change in compositionrather than particle size, and that a relative small change in thecomposition of the nanoparticle core can give rise to a substantialshift in selected electronic and optical properties. For example, theinvention provides a method for increasing the peak emission wavelengthof a luminescent semiconductor nanoparticle without necessarilyincreasing particle size or adversely impacting on brightness orcompatibility with a desired passivating overlayer, wherein (1) asignificant wavelength shift results from a relatively small change incomposition, and (2) the desired properties of the nanoparticle areretained.

The invention also provides a way to make a single overcoating material(e.g., ZnCdS) serve for many distinct nanocrystal core compositions. Inaddition to the direct savings in product development costs, additionalutility is achieved when a common overcoating is employed among multiplecore types because this results in a common platform for subsequentprocessing including solubilization and conjugation of the quantum dotsto biological content. Finally, use of a common overcoating approachwith several different core materials helps ensure relatively commonsensitivities among the materials toward environmental factors such aspH and salt content; this is important when the materials are usedtogether in multi-color applications.

The present invention enables additional advantages in several ways.First, a given emission wavelength can be achieved through a combinationof size and composition tuning which breaks the absolute tie betweenparticle size and emission wavelength and allows for the possibility oflarge and small particles with common emission maxima. Second, the factthat large changes in important properties such as emission wavelength,can be effected with very small compositional changes in (for example)chalcogenide content, means that an overcoating strategy used for thepure material is likely to be applicable to the lightly modifiedcomposition with little change in methodology. Specifically in oneincluded example (infra), small amounts of tellurium have been added toan essentially pure CdSe core, resulting in a large emission maximumshift to the red. The highly optimized ZnCdS overcoating layer incommercial use with pure CdSe particles could be applied to lightlymodified CdSeTe core with little change resulting in a particle withbrightness and stability very similar to that of CdSe, but with emissionmaximum closer to that expected for CdTe.

While not wishing to bound by theory, at least two important mechanismsare probably at play in this example. Emphasis can first be placed onthe idea of lattice match. In this case, it is the fact that the lowlevels of tellurium in the selenium matrix of CdSe probably have anegligible impact on the lattice properties of the core. It can beexpected that the overcoating optimized to lattice match the pure CdSecore will also lattice match the ternary core. Emphasis canalternatively be placed on the electronic properties of the system. Inthis case it is the fact that the small amount of additional telluriumwill have a large impact on those aspects of the particle electronicsresulting in low energy emission, but not on those which typicallypre-dispose telluride particles to oxidative chemical decomposition.

In a first embodiment, the invention provides a method for making aluminescent semiconductor nanoparticle N_(XAB) composed of X, A, and B,wherein X, A, and B are different, and the nanoparticle has a peakemission wavelength longer than a nanoparticle N_(XA) composed of X andA and longer than a nanoparticle N_(XB) composed of X and B, whereinN_(XA), N_(XB), and N_(XAB) are identically sized, the methodcomprising:

combining B with X and A in an amount such that the molar ratio B:(A+B)is in the range of approximately 0.01 to 0.10 and the molar ratioX:(A+B) is in the range of approximately 0.5:1.0 to 2:1, under reactionconditions effective to provide a nanoparticle.

In some contexts, it is desirable that a nanoparticle N_(XAB) composedof X, A, and B exhibit some properties similar to those of anidentically sized nanoparticle N_(XA) composed of X, A, and no B, andother properties of an identically sized nanoparticle N_(XB) composed ofX, B, and no A. For example, an N_(XA) nanoparticle contained within aparticular passivating overlayer may exhibit excellent brightness, goodchemical and photochemical stability, and breadth and efficiency ofexcitation, but emit at a wavelength shorter than that desired, while anN_(XB) nanoparticle of identical size may have a longer peak emissionwavelength but lack one or more of the other desirable properties, e.g.,compatibility with the overlayer material. The invention enables thepreparation of N_(XAB) nanoparticles that retain desirable properties ofN_(XA), particularly with regard to the suitability of a particularpassivating overlayer, but that have a peak emission wavelength closerto that of N_(XB) than N_(XA), without introduction of a significantamount of B. By a “suitable” passivating overlayer is meant an overlayerthat is well matched to the core in terms of lattice structure and thatis sufficiently protective (i.e., physically protective, protective ofelectronic properties, or both).

One application of the invention pertains to the preparation ofluminescent nanoparticles emitting strongly in the red and infraredspectral regions. Long wavelength light in these regions of theelectromagnetic spectrum is inefficiently scattered compared to bluerlight. Less scatter is associated with deeper penetration of light into,or more facile escape of light from, turbid media such as biologicaltissues or bulk powders (e.g. barrels of chemicals which need to bequality controlled). Red-emitting materials have been difficult tosynthesize and are not very common in nature; therefore, the environment(1) contributes a relatively small amount of background emission in thisregion of the spectrum, which is important for ultra-sensitivedetection, and (2) absorbs very little emitted light in this region,which leads to relatively brighter signals from far-red or infraredemitters. For biological samples in particular, this region allowsfluorescent detection with attenuated interference from water, blood, orintervening tissues such as skin.

Discrete size populations of cadmium telluride (CdTe) nanocrystalsfluoresce across this red spectral range, but are chemically unstable.Protective inorganic overlayers including CdS and ZnS have beensuccessfully applied to CdSe cores, but applying a sufficientlyprotective coating to CdTe, with its even longer bond lengths and higherconduction band energies, has not resulted in stable nanoparticles. Thepresent invention now enables preparation of nanoparticles that can belattice-matched to overlayer materials suitable for deposition on CdSecores and maintain electronic properties favorable to passivation withoverlayers useful with CdSe cores, but that, like CdTe, fluoresce in theredder spectral range. Addition of a small amount of tellurium to thesynthesis of the underlying cadmium selenide cores according to themethod of the invention results in unexpectedly large red shifts in theemission spectra for the composite materials. In fact, and asdemonstrated in the examples, nanoparticles composed of a CdSe core anda CdZnS overlayer (“CdSe/CdZnS” nanoparticles) modified so as to includeless than 5 mole percent of Te in the synthesis of the core exhibit apeak emission wavelength more than 100 nm outside of the conventionalwindow for pure CdSe, and are bright, stable nanoparticles emitting inthe red and infrared spectral ranges.

In a related embodiment, then, the invention provides a method forpreparing a nanoparticle that exhibits emission in the red or infraredspectral range which comprises:

(a) combining a cadmium precursor, a selenium precursor, and a telluriumprecursor in a solvent, wherein the mole ratio of tellurium to seleniumis the range of about 0.01 to about 0.1;

(b) inducing formation of a nanoparticle core using a first accelerant;

(c) combining the nanoparticle core with a solvent, a sulfur precursor,a second accelerant, and a cadmium precursor, a zinc precursor, or botha cadmium precursor and a zinc precursor.

In another embodiment, the invention provides a luminescentsemiconductor nanoparticle comprising a crystalline core and aprotective, passivating overlayer, wherein:

the core comprises Cd_(a)Se_(b)Te_(c) in which the ratio a:(b+c) is inthe range of about 0.7 to about 1.5 and the ratio c:(b+c) is in therange of about 0.001 to about 0.2; and

the overlayer layer comprises Cd_(x)Zn_(y)S_(z), wherein the ratio(x+y):z is in the range of about 0.7 to about 1.5.

In a further embodiment of the invention, a luminescent semiconductornanoparticle N_(XAB) is provided which comprises a first component X, asecond component A, and a third component B, and exhibiting an increasedpeak emission wavelength relative to that of a luminescent semiconductornanoparticle N_(XA) of identical size and comprised of X, A, and no B,wherein:

(a) N_(XAB) has a composition XA_(Z)B_(1-Z) wherein Z is in the range ofapproximately 0.900 to 0.999;

(b) X is selected from Cd, Zn, Hg, Al, Ga, In, and A and B are selectedfrom Se, Te, S, As, Sb, P, Pb, and Sn wherein (i) when X is Cd, Zn, orHg, then A and B are selected from Se, Te, and S, and (ii) when X is Al,Ga, or In, then A and B are selected from As, Sb, P, Pb, and Sn; and

(c) the increase in peak emission wavelength characterized by therelationship (1)|(λ_(XAB)−_(XA))|>|(λ_(XAB)−λ_(XB))|  (1)

in which:

λ_(XAB) is the peak emission wavelength of N_(XAB);

λ_(XA) is the peak emission wavelength of N_(XA); and

λ_(XB) is the peak emission wavelength of a luminescent, semiconductornanoparticle N_(XB) composed of X, B, and no A, and having the same sizeas N_(XAB) and N_(XA).

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a graph illustrating the temporal evolution of peak emissionwavelength over the course of typical nanocrystal core reactions. Thetwo lines represent the growth curves for cores that have been grownwith or without the addition to the synthesis of a small amount oftellurium, as described in Example 1.

FIG. 2 shows the transmission electron micrographs obtained on coresprepared in the absence of tellurium (FIG. 2A) or with 2.5 mole %tellurium (FIG. 2B). As may be seen, the CdSeTe cores in FIG. 2B are ofcomparably size as or even smaller than the CdSe cores in FIG. 2A, inspite of the fact that the CdSeTe cores exhibit a peak emissionwavelength of 734 nm, compared with an emission maximum of 630 nm fromthe CdSe cores.

FIG. 3 shows the final emission spectra obtained for an overlayered corestructure produced from cores containing 0% or 2.5% added tellurium, asdescribed in Example 2.

FIG. 4 shows transmission electron micrographs obtained for the coresprepared in Example 1, with 2.5 mole % tellurium, and for thecorresponding overlayered core structures prepared in Example 2.

DETAILED DESCRIPTION OF THE INVENTION Definitions and Overview

It must be noted that, as used in this specification and the appendedclaims, the singular forms “a”, “an” and “the” include plural referentsunless the context clearly dictates otherwise. Thus, for example,reference to “a component” or “an element” refers to a single componentor element as well as two or more components or elements, which may ormay not be combined in an admixture.

In describing and claiming the present invention, the followingterminology will be used in accordance with the definitions set outbelow.

The term “nanoparticle” refers to a particle, generally a semiconductiveparticle, having a diameter in the range of about 1-1000 nm, preferablyin the range of about 2-50 nm, more preferably in the range of about2-20 nm.

The terms “semiconductor nanoparticle” and “semiconductive nanoparticle”refer to a nanoparticle as defined herein, that is composed of aninorganic semiconductive material, an alloy or other mixture ofinorganic semiconductive materials, an organic semiconductive material,or an inorganic or organic semiconductive core contained within one ormore semiconductive overcoat layers.

The terms “semiconductor nanocrystal,” “quantum dot” and “Qdot®nanocrystal” are used interchangeably herein to refer to semiconductornanoparticles composed of an inorganic crystalline material that isluminescent (i.e., they are capable of emitting electromagneticradiation upon excitation), and include an inner core of one or morefirst semiconductor materials that is optionally contained within anoverlayer of a second inorganic material. The surrounding overlayermaterial will preferably have a bandgap energy that is larger than thebandgap energy of the core material and may be chosen to have an atomiclattice close to that of the core substrate. The “overlayer” materialmay be formed as an interfacial component of the core

The term “solid solution” is used herein to refer to a compositionalvariation that is the result of the replacement of an atom or ion withanother atom or ion, e.g., CdS in which some of the Cd atoms have beenreplaced with Zn. This is in contrast to a “mixture,” a subset of whichis an “alloy,” which is used herein to refer to a class of matter withdefinite properties whose members are composed of two or moresubstances, each retaining its own identifying properties.

By “luminescence” is meant the process of emitting electromagneticradiation (light) from an object. Luminescence results when a systemundergoes a transition from an excited state to a lower energy statewith a corresponding release of energy in the form of a photon. Theseenergy states can be electronic, vibrational, rotational, or anycombination thereof. The transition responsible for luminescence can bestimulated through the release of energy stored in the system chemicallyor added to the system from an external source. The external source ofenergy can be of a variety of types including chemical, thermal,electrical, magnetic, electromagnetic, and physical, or any other typeof energy source capable of causing a system to be excited into a statehigher in energy than the ground state. For example, a system can beexcited by absorbing a photon of light, by being placed in an electricalfield, or through a chemical oxidation-reduction reaction. The energy ofthe photons emitted during luminescence can be in a range fromlow-energy microwave radiation to high-energy X-ray radiation.Typically, luminescence refers to photons in the range from UV to IRradiation, and usually refers to visible electromagnetic radiation(i.e., light).

The term “monodisperse” refers to a population of particles (e.g., acolloidal system) wherein the particles have substantially identicalsize and shape. For the purpose of the present invention, a“monodisperse” population of particles means that at least about 60% ofthe particles, preferably about 75-90% of the particles, fall within aspecified particle size range. A population of monodisperse particlesdeviates less than 10% rms (root-mean-square) in diameter and preferablyless than 5% rms.

The phrase “one or more sizes of nanoparticles” is used synonymouslywith the phrase “one or more particle size distributions ofnanoparticles.” One of ordinary skill in the art will realize thatparticular sizes of nanoparticles such as semiconductor nanocrystals areactually obtained as particle size distributions.

By use of the term “narrow wavelength band” or “narrow spectrallinewidth” with regard to the electromagnetic radiation emission of thesemiconductor nanocrystal is meant a wavelength band of emissions notexceeding about 60 nm, and preferably not exceeding about 30 nm inwidth, more preferably not exceeding about 20 nm in width, and symmetricabout the center. It should be noted that the bandwidths referred to aredetermined from measurement of the full width of the emissions at halfpeak height (FWHM), and are appropriate in the emission range of200-2000 nm.

By use of the term “a broad wavelength band,” with regard to theexcitation of the semiconductor nanocrystal is meant absorption ofradiation having a wavelength equal to, or shorter than, the wavelengthof the onset radiation (the onset radiation is understood to be thelongest wavelength (lowest energy) radiation capable of being absorbedby the semiconductor nanocrystal). This onset occurs near to, but atslightly higher energy than the “narrow wavelength band” of theemission. This is in contrast to the “narrow absorption band” of dyemolecules, which occurs near the emission peak on the high energy side,but drops off rapidly away from that wavelength and is often negligibleat wavelengths further than 100 nm from the emission.

The terms “emission peak” and “peak emission wavelength” are usedinterchangeably to refer to the wavelength of light within thecharacteristic emission spectra exhibited by a particular semiconductornanocrystal size distribution that demonstrates the highest relativeintensity.

The term “alkyl” as used herein refers to a branched or unbranchedsaturated hydrocarbon group of 1 to approximately 24 carbon atoms, suchas methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl,octyl, decyl, tetradecyl, hexadecyl, eicosyl and tetracosyl, as well ascycloalkyl groups such as cyclopentyl and cyclohexyl. Similarly, alkanesare saturated hydrocarbon compounds such as methane, ethane, and soforth. The term “lower alkyl” is intended to mean an alkyl group of 1 to4 carbon atoms, and thus includes methyl, ethyl, n-propyl, isopropyl,n-butyl, isobutyl and t-butyl.

The term “alkene” as used herein refers to a branched or unbranchedhydrocarbon compound typically although not necessarily containing 2 toabout 29 carbon atoms and at least one double bond, such as ethylene,n-propylene, isopropylene, butene, butylene, propylene, octene,decylene, and the like. Generally, although not necessarily, the alkenesused herein contain 2 to about 29 carbon atoms, preferably about 8 toabout 20 carbon atoms. The term “lower alkene” is intended to mean analkene of 2 to 4 carbon atoms.

The term “alkyne” as used herein refers to a branched or unbranchedhydrocarbon group typically although not necessarily containing 2 toabout 24 carbon atoms and at least one triple bond, such as acetylene,allylene, ethyl acetylene, octynyl, decynyl, and the like. Generally,although again not necessarily, the alkynes used herein contain 2 toabout 12 carbon atoms. The term “lower alkyne” intends an alkyne of 2 to4 carbon atoms, preferably 3 or 4 carbon atoms.

The semiconductor nanoparticles of the invention are particles composedof a semiconductive material and having a diameter in the range ofapproximately 1 nm to 1000 nm, generally in the range of approximately 1to 50 nm, and optimally in the range of approximately 2 to 20 nm. Thesemiconductive material is an inorganic, crystalline material thatincludes at least three elements, which, in the formation of thenanoparticle, may be combined as individual elements and/or as alloys orother mixtures as will be discussed in further detail below. Thesemiconductor nanoparticles of the invention all exhibit luminescence.As is well understood in the art, luminescence results when a systemundergoes a transition from an excited state to a lower energy statewith a corresponding release of energy in the form of a photon. Theseenergy states can be electronic, vibrational, rotational, or anycombination thereof. The transition responsible for luminescence can bestimulated through the release of energy stored in the system chemicallyor added to the system from an external source. The external source ofenergy can be of a variety of types including chemical, thermal,electrical, magnetic, electromagnetic, and physical, or any other typeof energy source capable of causing a system to be excited into a statehigher in energy than the ground state. For example, a system can beexcited by absorbing a photon of light, by being placed in an electricalfield, or through a chemical oxidation-reduction reaction. The energy ofthe photons emitted during luminescence can range from low energymicrowave radiation to high energy x-rays; luminescence in the presentcontext refers to emission in the visible and infrared spectral regions.To ensure chemical and photochemical stability, enhance brightness, andprovide physical protection of the nanoparticle, the nanoparticleitself, in most contexts, is contained within a suitable passivatingoverlayer. In contrast to the semiconductive core, the material used forthe passivating layer has to be electronically insulating whilenevertheless sufficiently “matched” in lattice structure to the coresurface to remain intimately wed thereto. In addition, as discussedinfra and in U.S. Pat. No. 6,815,064 to Treadway et al., it is desirablefor the overlayer material to be used to dope the core material, andvice versa, in the region of the core/overlayer interface.

In a first embodiment, the invention provides a method for making aluminescent semiconductor nanoparticle N_(XAB) composed of X, A, and B,wherein X, A, and B are different, wherein the nanoparticle has a peakemission wavelength longer than that of a nanoparticle N_(XA) composedof X and A (and no B) and longer than that of a nanoparticle N_(XB)composed of X and B (and no A), wherein N_(XA), N_(XB), and N_(XAB) areidentically sized. For instance, when X is Cd, A is Se, and B is Te, thenanoparticle N_(XAB) has a peak emission wavelength greater than that ofeither CdSe or CdTe. The method involves combining B with X and A, inamounts such that the molar ratio B:(A+B) is in the range ofapproximately 0.01 to 0.10 and the molar ratio X:(A+B) is in the rangeof approximately 0.5:1.0 to 2:1, under reaction conditions effective toprovide a nanoparticle. In a preferred embodiment, the molar ratioB:(A+B) is in the range of approximately 0.01 to 0.05, and the ratioX:(A+B) is approximately 1:1.

Many inorganic materials are suitable herein, and the invention is notlimited in this regard. In one embodiment, by way of example, X is aGroup 12 element, A is a Group 16 element, and B is an element selectedfrom Groups 12, 13, 14, 15, and 16, with all group numbers referring tothe IUPAC notation system for numbering element groups, as set forth inthe Handbook of Chemistry and Physics, 81^(st) Edition (CRC Press,2000). In this embodiment, X is Cd, Zn, or Hg, and A and B arepreferably selected from Se, Te, and S. For instance, as noted above, Xmay be Cd, A may be Se, and B may be Te, such that XA is CdSe, XB isCdTe, and N_(XAB) comprises Cd, Se, and Te. In another embodiment, X isa Group 13 element and A is a Group 15 element. In this embodiment, X ispreferably Al, Ga, or In, and A and B are selected from As, Sb, and P.

One advantage of the present invention is that it is possible toengineer the peak emission wavelength without increasing nanoparticlesize or changing the composition significantly. That is, the amount of Bincorporated in the aforementioned method represents at most 10 mole %,preferably at most 5 mole %, of the combined quantities of A and B. Yet,while the peak emission wavelength λ_(XA) of a nanoparticle composed ofX, A, and no B may be shorter than the peak emission wavelength λ_(XB)of a identically sized nanoparticle composed of X, B, and no A, the peakemission wavelength of an identically sized N_(XAB) nanoparticle mayactually be longer than λ_(XB). By “identically sized” is meant that thenanoparticles have essentially the same diameter. If desired, however,the peak emission wavelength λ_(XAB) of N_(XAB) may be engineered to beshorter than the peak emission wavelength λ_(XB) of an identically sizednanoparticle N_(XB). Generally, although not necessarily, the differencebetween λ_(XAB) and λ_(XB) will be less than the difference betweenλ_(XAB) and λ_(XA).

In certain embodiments, the peak emission wavelength λ_(XAB) of N_(XAB)is at least 50 nm longer than the peak emission wavelength λ_(XA) of ananoparticle N_(XA) composed of X, A, and no B. For N_(XAB) prepared soas to have a peak emission wavelength shifted toward the red to infraredregions of the electromagnetic spectrum, λ_(XAB) is greater than 600 nm,preferably greater than 650 nm. In Example 1, for instance,incorporating 2.5 mole % Te into CdSe nanoparticle cores having aparticle diameter of approximately 5 nm results in a shift in wavelengthfrom about 630 nm to about 734 nm.

In combining B with X and A to form the nanoparticles N_(XAB), B, X, andA may take the form of the individual elements, but more typically willcomprise forms of those elements that essentially serve as precursors.For example, when X is Cd, the compound employed in the aforementionedmethod may be an organometallic compound such as Cd(CH₃)₂, an oxide suchas CdO, a halogenated compound such as CdCl₂, or cadmium salts such ascadmium acetate, cadmium acetoacetonate, and cadmium nitrate. Similarly,the compounds providing A and B are normally in the tri-n-alkylphosphineadducts such as tri-n-(butylphosphine)selenide (TBP-Se) andtri-n-(octylphosphine)selenide (TOP-Se), hydrogenated compounds such asH₂Se, silyl compounds such as bis(trimethylsilyl)selenium ((TMS)₂Se),and metal salts such as NaHSe. These are typically formed by combining adesired element, such as Se, with an appropriate coordinating solvent,e.g., tri-n-octylphosphine (TOP). Other exemplary organic precursors aredescribed in U.S. Pat. Nos. 6,207,299 and 6,322,901 to Bawendi et al.,and synthetic methods using weak acids as precursor materials aredisclosed by Qu et al., (2001) “Alternative Routes toward High QualityCdSe Nanocrystals,” Nano Lett., 1(6):333-337, the disclosures of whichare incorporated herein by reference. Thus, suitable chemical compounds,or “precursors,” for introducing and combining X, A, and B in a reactionmixture include, but are not limited to, Group 16 elements;trialkylphosphines of Group 16 elements (such astri-n-butylphosphine-substituted Se); bis-trialkylsilyl-substitutedGroup 16 elements (such as bis(trimethylsilyl)selenide); Groups 12, 13,and 14 metals and metal salts of acids, such as nitrates, acetates andcarbonates; Groups 12, 13, and 14 metal oxides; and C₁-C₄alkyl-substituted Groups 12, 13, and 14 metals. Exemplarynon-organometallic precursors and synthetic methods using suchprecursors are described in U.S. Pat. No. 6,576,291 to Bawendi et al.

The reaction used to prepare N_(XAB) nanoparticles is carried out in acoordinating solvent, optionally diluted with an essentiallynon-coordinating solvent (e.g., an alkane). Suitable coordinatingreaction solvents include, by way of illustration and not limitation,amines, phosphines, phosphine oxides, alkenes, alkynes, fatty acids,ethers, furans, pyridines, and combinations thereof (in the latter case,the reaction solvent termed a “solvent system”). Examples ofrepresentative solvents are as follows: (a) amine solvents—alkylaminessuch as dodecylamine and hexyldecylamine; (b) phosphines—alkylphosphines, particularly trialkyl phosphines such as tri-n-TBP and TOP;(c) phosphine oxides—alkyl phosphine oxides, particularly trialkylphosphine oxides such as tri-n-octylphosphine oxide (TOPO); fattyacids—stearic and lauric acids; ethers and furans—tetrahydrofuran andits methylated forms, glymes; phosphoacids—hexylphosphonic acid,tetradecylphosphonic acid, and octylphosphinic acid, preferably used incombination with an alkyl phosphine oxide such as TOPO; andpyridines—pyridine per se, alkylated pyridines, nicotinic acid, etc.Coordinating solvents can be used alone or in combination. TOP-TOPOsolvent systems are commonly utilized in the art, as are other related(e.g., butyl) systems. For example, TOP and TOPO can be used incombination to form a cadmium solution, while TOP, alone, can be used toform a selenium solution.

Technical grade coordinating solvents can be used, and benefits can beobtained from the existence of beneficial impurities in such solvents,e.g. TOP, TOPO or both. However, in one preferred embodiment, thecoordinating solvent is pure. Typically this means that the coordinatingsolvent contains less than 10 vol. %, and more preferably less than 5vol. % of impurities that can function as reductants. Therefore,solvents such as TOPO at 90% or 97% purity and TOP at 90% purity areparticularly well suited for use in the methods of the invention.

The method used to prepare N_(XAB) nanoparticles may be carried out inthe presence of an accelerant, either heat, a chemical reaction promoter(as described in U.S. Patent Publication No. US 2003/0097976 A1 toZehnder et al., of common assignment herewith), or both in order toincrease the rate of reaction. As described in the aforementioned patentpublication, the reaction promoter can be an oxygen source or a reducingagent. Phosphine-based reductants are a preferred class of reducingagents. Non-phosphine, non-ligating chemical reductants such ashydroquinone, however, are also suitable. Illustrative reducing agentsuseful in conjunction with the present method include tertiary,secondary, and primary phosphines (e.g., diphenylphosphine,dicyclohexylphosphine, and dioctylphosphine), amines (e.g., decylamineand hexadecylamine), hydrazines, hydroxyphenyl compounds (e.g.,hydroquinone and phenol), hydrogen, hydrides (e.g., sodium borohydride,sodium hydride and lithium aluminum hydride), metals (e.g., mercury andpotassium), boranes (e.g., THF:BH₃ and B₂H₆), aldehydes (e.g.,benzaldehyde and butyraldehyde), alcohols and thiols (e.g., ethanol andthioethanol), reducing halides (e.g., I⁻ and I₃ ⁻), polyfunctionalreductant versions of these species (e.g., a single chemical speciesthat contains more than one reductant moiety, each reductant moietyhaving the same or different reducing capacity, such astris(hydroxypropyl)phosphine and ethanolamine), and so forth. In thepreferred embodiment herein, wherein the nanoparticle core andpassivating overlayer comprise Cd, Zn, Se, Te, or S, reducing agentshould, correspondingly, be capable of reducing cadmium(II), zinc(II),selenium(0), tellurium(0), or sulfur(0).

As discussed above, the present nanoparticles are generally providedwith a passivating overlayer wed to the nanoparticle surface with aninterfacial region formed at the juncture of the nanoparticle and theoverlayer. The interfacial region may be discontinuous, comprise amonolayer, or comprise many monolayers, and the region may incorporateseveral combinations of elements. For example, in a nanocrystal with aCdSe core and a ZnS overlayer, the interfacial region might include thecombinations Cd/Zn/S, Cd/Se/Zn, or even Cd/Se/Zn/S. The region may alsocontain elements not native to either the core or overlayer structures.For example in the CdSe/ZnS/Cd case, small numbers of oxygen atoms mightbe introduced into the interfacial region during synthesis. Otherelements that may be used as additives include Fe, Nb, Cr, Mn, Co, Cu,and Ni. There may also be a solid solution gradient in which, forexample, in a CdSe/ZnS nanoparticle, the passivating overlayer containsmostly Zn and S at its exterior surface with some Cd and Se graduallyintroduced as the distance to the nanoparticle core decreases; the samesolid solution gradient can be introduced into the nanoparticle core,with increased amounts of Zn and S present as the distance to the coresurface decreases.

In a preferred embodiment, the interfacial region is in the form of asolid solution with a gradient as just described, wherein the region iscomprised of some or all of the chemical elements from the passivatingoverlayer and the core, and may also include an additive as described incommonly assigned U.S. Pat. No. 6,815,064 to Treadway et al., previouslycited and incorporated by reference herein. Conveniently, in thisembodiment, the method is carried out as a one-pot synthesis, asdescribed in the aforementioned patent. In this method, the nanoparticlecores are prepared as described above, but are not isolated immediately;rather, a selected additive is added to the solution containing thecores, along with precursors to the elements that will form thepassivating overlayer. The additive is generally comprised of a materialselected from Groups 2, 12, 13, 14, 15, and 16, and may also be found inthe nanoparticle core. Further detail concerning the synthesis ofnanoparticles in this way may be found in the aforementioned patent toTreadway et al.

The passivating overlayer of the luminescent nanoparticles may becovered with an organic or other outer layer. The outer layer may becomprised of materials selected to provide compatibility with asuspension medium, such as a short-chain polymer terminating in a moietyhaving affinity for a suspending medium, or with a moiety that possessesaffinity for a particular surface. Common materials suitable for theouter layers include, but are not limited to, polystyrene, polyacrylate,polyimide, polyacrylamide, polyethylene, poly(phenylenevinylene),polypeptides, polysaccharides, polysulfone, polypyrrole, polyimidazole,polythiophene, polyether, epoxies, silica glass, silica gel, titania,siloxanes, polyphosphate, hydrogels, agarose, celluloses, and the like.The coating can be in the range of about 2 to 100 nm thick, preferably 2to 10 nm thick.

It is important to note that the nanoparticles exhibit high quantumyields upon dispersion in a solvent system, with the quantum yield quitestable over time and as a function of material handling. Many commonuses of quantum dots require dispersion in aqueous solvent systems, inturn requiring surface modification of oil-dispersible quantum dots toenable dispersion in aqueous media. U.S. Pat. No. 6,649,139 to Adams etal., incorporated by reference herein, describes an optimal surfacemodification method in which mixed hydrophobic/hydrophilic polymertransfer agents are bound to the surface of the quantum dots. Thenanoparticles of the present invention exhibit a high, stable quantumyield not only in a dispersion in an organic solvent system but also inan aqueous dispersion. As indicated in the example, the nanoparticles ofthe invention not only become much brighter initially as a function ofsmall decreases in the amount of added tellurium (“B”), but they alsomaintain this brightness even after dispersal in water. In general, thequantum yield for a dispersion of N_(XAB) nanoparticles in either anorganic solvent system or an aqueous solvent system is greater than0.10, preferably greater than 0.40, and most preferably greater than0.80.

In a related embodiment, the invention provides a method for preparing ananoparticle that exhibits emission in the red or infrared spectralrange by: combining a cadmium precursor, a selenium precursor, and atellurium precursor in a solvent, wherein the mole ratio of tellurium toselenium is the range of about 0.01 to about 0.10, preferably in therange of about 0.01 to about 0.05; inducing formation of a nanoparticlecore using a first accelerant; optionally isolating the nanoparticlecore; and combining the nanoparticle core with a solvent, a sulfurprecursor, a second accelerant, and a cadmium precursor, a zincprecursor, or both a cadmium precursor and a zinc precursor. Cadmiumprecursors, selenium precursors, and tellurium precursors are asdescribed above, and a preferred solvent is a phosphine, a phosphineoxide, an alkene, an alkyne, an amine, or a combination thereof, withrepresentative such solvents described hereinabove. In one embodiment,at least one of the first accelerant and the second accelerant is heat,and in another embodiment at least one of the first accelerant and thesecond accelerant is a chemical promoter, i.e., an oxygen source or areducing agent, as described above. In a particularly preferredembodiment, a chemical promoter is combined with heat, and the reactionis carried out in the presence of a phosphonic acid, a phosphinic acid,a carboxylic acid, a sulfonic acid, or a combination thereof.

In a further embodiment, a luminescent semiconductor nanoparticle isprovided that comprises a crystalline core and a protective, passivatingoverlayer, wherein the core comprises Cd_(a)Se_(b)Te_(c) in which theratio a:(b+c) is in the range of about 0.7 to about 1.5 and the ratioc:(b+c) is in the range of about 0.001 to about 0.20, and the overlayercomprises Cd_(x)Zn_(y)S_(z), wherein the ratio (x+y):z is in the rangeof about 0.7 to about 1.5 and x or y may be zero. The overlayer mayfurther comprise Se and/or Te, and the Se and Te may be distributedthroughout the core, or may be present in distinct regions within thecore. The emission quantum yield of an aqueous dispersion of theseparticles, prepared according to the method of U.S. Pat. No. 6,649,139to Adams et al., cited above, is at least 0.1, preferably at least 0.4,and most preferably at least 0.8.

It is to be understood that while the invention has been described inconjunction with specific embodiments thereof, the foregoing descriptionas well as the examples that follow are intended to illustrate and notlimit the scope of the invention. Other aspects, advantages, andmodifications within the scope of the invention will be apparent tothose skilled in the art to which the invention pertains. All patents,patent applications, patent publications, journal articles, and otherreferences cited herein are incorporated by reference in theirentireties.

EXPERIMENTAL

The following examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how tocarry out the method of the invention and make the presentnanoparticles. Efforts have been made to ensure accuracy with respect tonumbers (e.g., amounts, temperature, etc.) but some experimental errorand deviations should, of course, be allowed for. Unless indicatedotherwise, parts are parts by weight, temperature is in degreescentigrade and pressure is at or near atmospheric.

Materials: In the following examples, materials were obtained asfollows: tetradecylphosphonic acid (TDPA, 98%), selenium shot (99.999%),and tellurium shot (99.9999%), from Alfa; tri-n-octylphosphine oxide(TOPO, 99%), tri-n-octylphosphine (TOP, 97%), diethylzinc (100%), andanhydrous cadmium acetate, from Strem; diphenylphosphine, from Aldrich;and hexmethyldisilthiane, from Fluka.

Example 1 Synthesis of Mixed Cadmium, Selenium, TelluriumNanocrystalline Cores

TDPA (0.549 g), TOPO (6.000 g), and a magnetic stir bar were added to aclean, dry 50 mL three-neck round bottom flask. Port 1 of the flask wasequipped with a gas inlet adapter to allow for evacuation and nitrogenrefill, port 2 was equipped with a temperature probe attached to atemperature control device, and port 3 was fitted with a rubber septum.The flask was evacuated, refilled with nitrogen gas, and maintainedunder a nitrogen blanket. TOP (3.6 mL) and 1.972 g of Cd-TOP solution(containing cadmium acetate dissolved in TOP at a concentration of 0.5moles of cadmium per kg solution) were added. A needle was inserted intothe septum on port 3. The mixture was heated to 260° C. and held at thattemperature for 20 minutes. The needle was removed and the reactionflask was heated to 355° C. During this heating step, diphenylphosphine(0.030 mL) was added, and at 340° C., a solution of 0.035 mL of TOP-Te(1M tellurium shot in TOP) and 1.346 mL of TOP-Se (1M selenium shot inTOP) was added. Aliquots were removed every 30 seconds to determine thecurrent emission maximum. The reaction was halted by addition of 4.0 mLof room-temperature TOP.

A graph of the peak emission wavelength obtained versus time is shown inFIG. 1. The second curve, identified in the figure as “0% Te,”corresponds to cores prepared in a manner identical to that justdescribed, but wherein the TOP-Te is replaced with an equivalent molaramount of TOP-Se. As may be seen in the graph, the introduction of Teinto the CdSe core at a level approximately 2.5 mole % (i.e.,Te/(Se+Te)=0.025) resulted in an increase in peak emission wavelength ofapproximately 100 nm.

FIG. 2 provides transmission electron micrographs (TEMs) obtained oncores prepared in the presence of either no tellurium (FIG. 2A) or 2.5mole % tellurium (FIG. 2B). As may be seen, the CdSeTe cores in FIG. 2Bare of comparable size as or even smaller than the CdSe cores in FIG.2A, in spite of the fact that the CdSeTe cores exhibit a final peakemission wavelength of 734 nm, compared with a final emission maximum of630 nm from the CdSe cores.

Example 2 Synthesis of Mixed Cadmium, Selenium, TelluriumNanocrystalline Cores with Passivating Overlayer

TOPO (3.383 g) and a magnetic stir bar were added to a 50 mL three-neckround bottom flask. Ports 1, 2, and 3 were capped as described inExample 1. The flask was evacuated and refilled with nitrogen gas. Undervacuum and with constant stirring, the TOPO was heated to 180° C. for 1hour. The flask was refilled with nitrogen gas and allowed to cool to100° C. before TOP (3.4 mL) was added. Ethanol (21.3 mL) and a sample ofthe cores prepared in Example 1 (10.7 mL warmed to 50° C.) were added toa 60 mL centrifuge tube. The mixture was centrifuged, the supernatantwas discarded, and the pellet was redispersed in hexanes. The dispersionwas then added to the reaction flask at 100° C. A vacuum was applied toremove the hexanes leaving the nanocrystals dispersed in TOPO and TOP.Decylamine (2.8 mL) was added to the reaction flask. In a second flask,a mixture of 3.905 g of Cd-TOP solution described in Example 1, TDPA(2.730 g), and TOP (2.7 mL) were momentarily heated to 250° C. and thencooled to 100° C. under a nitrogen atmosphere. After 45 minutes, 2.5 mLof the cadmium acetate/TDPA/TOP solution was added to the cores. Thereaction flask was heated to 230° C. and 3.9 mL of a solution containingTOP (2.926 g), diethylzinc (0.187 g), and hexamethyldisilthiane (0.203g) was added drop-wise over a period of three hours. Aliquots werewithdrawn from the reaction periodically to track the emissionproperties.

The final emission spectrum obtained for these “core/overlayer”structures is shown in FIG. 3. The second spectrum shown corresponds tothat obtained for the “0% Te” nanoparticle cores (described inExample 1) provided with a passivating overlayer as described above.

Additionally, it was found that when only small amounts of telluriumwere needed to effect a substantial wavelength shift, extremely thick,high quality shells could still be grown atop the cores. This result isillustrated in FIG. 4, which provides transmission electron micrographsobtained for the cores prepared in Example 1, with 2.5 mole % tellurium(FIG. 4A), and for the corresponding “core/overlayer” structuresprepared in Example 2 (FIG. 4B).

Example 3 Evaluation of Quantum Yield

The nanoparticles prepared in Example 2 were modified as described inU.S. Pat. No. 6,649,139 to Adams et al. to prepare the particles fordispersal in an aqueous solvent system, as follows.

(a) Synthesis of Hydrophobically Modified Hydrophilic Polymers forAttachment to the Nanoparticle Surface:

A modified polyacrylic acid was prepared by diluting 100 g [0.48 molCOONa] of poly(acrylic acid, sodium salt) (obtained from Aldrich,molecular weight 1200) two-fold in water and acidifying in a 1.0 L roundbottom flask with 150 ml (1.9 mol) of concentrated HCl. The acidifiedpolymer solution was concentrated to dryness on a rotary evaporator (100mbar, 80° C.). The dry polymer was evacuated for 12 hours at <10 mbar toensure water removal. A stirbar and 47.0 g (0.24 mol) of1-[3-(dimethyl-amino)-propyl]-ethylcarbodiimide hydrochloride(EDC-Aldrich 98%) were added to the flask, and the flask was then sealedand purged with N₂, and fit with a balloon. 500 ml of anhydrousN,N-dimethylformamide (Aldrich) was transferred under positive pressurethrough a cannula to this mixture; and the flask was swirled gently todissolve the solids. 32 ml (0.19 mol) of octylamine was transferreddropwise under positive pressure through a cannula from a sealedoven-dried graduated cylinder into the stirring polymer/EDC solution,and the stirring continued for 12 hours. This solution was concentratedto <100 ml on a rotary evaporator (30 mbar, 80° C.), and the polymer wasprecipitated by addition of 200 ml di-H₂O to the cooled concentrate,which produced a gummy white material. This material was separated fromthe supernatant and triturated with 100 ml di-H₂O three more times. Theproduct was dissolved into 400 ml ethyl acetate (Aldrich) with gentleheating, and basified with 200 ml di-H₂O and 100 gN—N—N—N-tetramethylammonium hydroxide pentahydrate (0.55 mo) for 12hours. The aqueous layer was removed and precipitated to a gummy whiteproduct with 400 ml of 1.27 M HCl. The product was decanted andtriturated with 100 ml of di-H₂O twice more, after which the aqueouswashings were back-extracted into 6×100 ml portions of ethyl acetate.These ethyl acetate solutions were added to the product flask, andconcentrated to dryness (100 mbar, 60° C.). The crude polymer wasdissolved in 300 ml of methanol and purified in two aliquots over LH-20(Amersham-Pharmacia-5.5 cm×60 cm column) at a 3 ml/minute flow rate.Fractions were tested by NMR for purity, and the pure fractions werepooled, while the impure fractions were re-purified on the LH-20 column.After pooling all of the pure fractions, the polymer solution wasconcentrated by rotary evaporation to dryness, and evacuated for 12hours at <10 mbar. The product was a white powder (25.5 g, 45% oftheoretical yield), which showed broad NMR peaks in CD₃OD [δ=3.1 b(9.4), 2.3 b (9.7), 1.9 1.7 1.5 1.3 b (63.3) 0.9 bt (11.3)], and clearIR signal for both carboxylic acid (1712 cm⁻¹) and amide groups (1626cm⁻¹, 1544 cm⁻¹).

(b) Preparation of Surface-Modified Nanocrystals:

Twenty milliliters of 3-5 μM (3-5 nmoles) of TOPO/TOP coatednanoparticles, prepared in Example 2, were precipitated with 20milliliters of methanol. The flocculate was centrifuged at 3000×g for 3minutes to form a pellet of the nanocrystals. The supernatant wasthereafter removed and 20 milliliters of methanol was again added to theparticles. The particles were vortexed to loosely disperse theflocculate throughout the methanol. The flocculate was centrifuged anadditional time to form a pellet of the nanocrystals. Thisprecipitation/centrifugation step was repeated an additional time toremove any excess reactants remaining from the nanocrystal synthesis.Twenty milliliters of chloroform were added to the nanocrystal pellet toyield a freely dispersed sol.

300 milligrams of the hydrophobically modified poly(acrylic acid) wasdissolved in 20 ml of chloroform. Tetrabutylammonium hydroxide (1.0 M inmethanol) was added to the polymer solution to raise the solution to pH10 (pH was measured by spotting a small aliquot of the chloroformsolution on pH paper, evaporating the solvent and thereafter wetting thepH paper with distilled water). Thereafter the polymer solution wasadded to 20 ml of chloroform in a 250 ml round bottom flask equippedwith a stir bar. The solution was stirred for 1 minute to ensurecomplete admixture of the polymer solution. With continued stirring thewashed nanocrystal dispersion described above was added dropwise to thepolymer solution. The dispersion was then stirred for two minutes toensure complete mixing of the components and thereafter the chloroformwas removed in vacuo with low heat to yield a thin film of theparticle-polymer complex on the wall of the flask. Twenty milliliters ofdistilled water were added to the flask and swirled along the walls ofthe flask to aid in dispersing the particles in the aqueous medium. Thedispersion was then allowed to stir overnight at room temperature. Atthis point the nanocrystals were freely dispersed in the aqueous medium.

Additional nanoparticles were prepared as in Example 2, but withdiffering amounts of added tellurium: 1%, 5%, 10%, and 100% (molepercent relative to the combined amounts of Se and Te). The quantumyield of each nanoparticle was evaluated before and after dispersal inan aqueous solvent system. The results are shown in Table 1:

TABLE 1 Comparison of Emission Quantum Yields (QY) as a Function ofAdded Tellurium % Te Added Core/Shell to Core Emission λ QY_(organic)QY_(water) 1 691 0.94 0.84 2.5 755 0.94 0.78 5 808 0.55 0.40 10 868 0.290.24 100 725 0.81 0.01

The result in surprising but clear: these quantum dots not only becamebrighter initially as a function of small decreases in the amount ofadded tellurium, but also maintained this brightness even afterdispersal in water. The invention thus enables the preparation ofexceptionally stable composite core/overlayer nanoparticles.

We claim:
 1. An aqueous dispersion of nanocrystals, comprising apopulation of luminescent semiconductor nanocrystals dispersed in anaqueous solvent system, wherein each nanocrystal in the populationcomprises a crystalline CdSeTe core, wherein the core comprises about1-10 mole % Te relative to the combined amounts of Se and Te, and aprotective, passivating overlayer comprising ZnCdS.
 2. The aqueousdispersion of claim 1, wherein the emission quantum yield is 0.40 to0.84.
 3. The aqueous dispersion of claim 1, wherein the emission quantumyield is 0.80 to 0.84.
 4. The aqueous dispersion of claim 1, wherein thepopulation of nanocrystals exhibits infrared emission.